The invention relates to a process for heating a soot filter in an exhaust gas system of an internal combustion engine, particularly a diesel engine, with at least one catalytic converter and a soot filter mounted downstream from the engine for accumulation of the soot.
The use of carbon filters in the exhaust gas systems of diesel engines is of the state of the art. In order to ensure the efficiency of the carbon filter it is necessary replace or recondition this filter from time to time. Burning of the carbon accumulated in the carbon filter for the purpose of reconditioning is of the state of the art. The temperature in the carbon filter required for this purpose in a diesel engine, if it exists at all, can be introduced into the engine only at great cost. DE 196 18 397 A1, for example, discloses moistening of the accumulated carbon with additional fuel and accordingly lowering of the temperature required. This entails not only additional expense for delivery of the fuel but also application of additional measures to prevent undesirable ignition in the carbon filter area.
The invention pursues the object of low-cost heating of a carbon filter in the exhaust gas system of an internal combustion engine, a diesel engine in particular, for the purpose of initiating burning of the carbon.
It is claimed for the invention that this object is attained by the process of the present invention for heating a carbon filter in the exhaust gas system of an internal combustion system, of a diesel engine in particular, with at least one catalytic converter and a carbon filter mounted downstream from this converter for accumulation of carbon as specified in claim 1, an exhaust gas system such that the catalytic converter mounted upstream from the carbon filter is heated to the point that the amount of heat introduced from the catalytic converter into the carbon filter is introduced into the carbon filter to the extent that combustion of the carbon is initiated. The heat to be introduced for initiation of carbon combustion may as a result be produced by simple means structurally near the carbon filter and independently of the layout and design of the engine. Reliable processes already disclosed for the heating of catalytic converters may be applied for heating. Even in the case of diesel engines with their low aptitude for adequate introduction of heat from the engine into the carbon filter, the process is simple and reliable in application without introduction of additional fuel into the carbon filter. The thermal losses are slight because of the structural proximity.
In a second embodiment, preference is to be given the process for heating a carbon filter in an exhaust gas system of an internal combustion engine, a diesel engine in particular, with at least one catalytic converter and a carbon filter mounted downstream from the carbon filter in this catalytic converter for accumulation of carbon as in which the catalytic converter mounted upstream from the carbon filter is heated by exothermal reaction in the catalytic converter mounted upstream from the carbon filter to the extent that the amount of heat introduced from the catalytic converter into the carbon filter heats the carbon filter to the point that combustion of the carbon is initiated. The heat initiating carbon combustion may as a result be generated directly in the exhaust gas system mounted upstream from the carbon filter in the structural vicinity of the carbon filter and independently of the layout and design of the engine and of additional heating means. The resulting thermal losses are especially small.
Especially advantageous is the process for heating of a carbon filter in an exhaust gas system of an internal combustion engine, a diesel engine in particular, with several catalytic converters mounted in the direction of exhaust gas flow and a carbon filter mounted downstream from them for accumulation of the carbon, as specified in a further embodiment, the catalytic converter mounted downstream in the direction of flow and immediately upstream from the carbon filter being heated to the point that the exothermal reaction is transferred from a catalytic converter mounted upstream in the direction of flow, in particular for initiation of detoxification of the catalytic converter mounted downstream from it, to the catalytic converter mounted downstream, and the heat from the catalytic converter mounted downstream in the direction of flow and mounted immediately upstream from the carbon filter being introduced into the carbon filter. In this way heating of the carbon filter mounted downstream but upstream from the catalytic converter is effected by design. Energy losses due to introduction of thermal energy by heat transfer and heat conduction from the exterior and the associated dangers of overheating of other components, upstream catalytic converters in particular, are prevented. Since heating results exclusively from transfer of the exothermal reaction from the catalytic converter mounted upstream to the catalytic converter mounted downstream, the only reactions used are those which take place in any event in conventional gas cleaning by two catalytic converters mounted in sequence in the direction of flow. Consequently, the catalytic converter mounted downstreamxe2x80x94and the carbon filter mounted downstream from itxe2x80x94may be heated for introduction of carbon combustion in a very simple manner, with no additional reactions and with no additional means required for limiting the temperature of other components. The process permits very low energy consumption and accordingly low-consumption engines.
In a further embodiment, the displacement may be effected as desired by simple means through xcex value control with an alternating rich/lean operating cycle of the internal combustion engine and accordingly heating of the downstream catalytic converter. Exhaust emissions may be kept at a low level under control as desired.
In still a further embodiment, the xcex value is adjusted for heating so that rich-mixture operation with xcex less than 1 is always maintained somewhat longer than the time allowing the O2 in the oxygen reservoir of the catalyst mounted upstream to convert the pollutants HC and CO, so that the conversion is effected at least in part by the O2 stored in the oxygen reservoir of the catalytic converter mounted downstream, and wherein the two oxygen reservoirs are refilled during lean-mixture operation with xcex greater than 1, which makes possible very simple transfer of reduction of the pollutants HC and CO carried out in the catalytic converter mounted upstream for cleaning of the gas in normal operation to the catalytic converter mounted downstream, as a result of which the catalytic converter mounted downstream is heated. The pollutants may undergo decomposition unchanged.
By preference the xcex value is regulated for heating so that for xcex the statement 0.95xe2x89xa7xcexxe2x89xa70.9 is valid during heating. As a result of the operation with a very rich mixture, the exothermal reaction is transferred in a short time to the catalytic converter mounted downstream, so that the required temperature is reached in a short time.
In a further embodiment, control of the rich-lean operating cycle is exerted by means of O2 sensors, xcex probes in particular, which check the exhaust gas in a position downstream from the catalytic converter, which permits highly sensitive regulation of the rich-mixture/lean-mixture operating cycle and accordingly control of both heating and exhaust gas composition. In other embodiments, the exhaust gas boundary values can be dependably adhered to and yet temperature increase safe for operation may be achieved by simple means.
In yet a further embodiment, the upstream catalyst is an oxidation catalyst and the downstream catalytic converter is a NOx adsorber wherein the NOx adsorber is heated, in particular to the desulfatization temperature, by shifting the exothermal reaction from the oxidation catalyst (3) to the NOx adsorber, which makes it possible to heat the carbon filter for initiation of carbon combustion without overheating of the oxidation catalyst.
In still a further embodiment, which the upstream catalyst is an oxidation catalyst and the downstream catalytic converter is a three-way catalytic converter with oxygen reservoir, wherein the three-way catalytic converter is heated by shifting the exothermal reaction from the first oxidation catalyst to the three-way catalytic converter, which makes it possible to promote carbon generation very advantageously by NOx formation.